1. Field
The disclosed concept pertains generally to adjustable frequency drive (AFD) systems and, more particularly, to such AFD systems, which control the speed, torque, horsepower and/or direction of an induction machine or other rotating electrical apparatus, such as, for example, an AC motor or generator. The disclosed concept also pertains to AFD structures for power conversion in generation applications.
2. Background Information
An adjustable frequency drive (AFD) system can be employed in a wide range of commercial applications, such as, for example and without limitation, HVAC, fans, pumps, conveyors, material handling and processing equipment, and other general industries, such as, for example and without limitation, forest products, mining, metals and printing.
If the stator terminals of an induction machine are connected to a three-phase AFD system, then the rotor will rotate in the direction of the stator rotating magnetic field. This is the induction machine motoring mode of operation. When load torque is applied to the motor shaft, the steady state speed is less than the synchronous speed.
When the induction machine speed is higher than the synchronous speed, and the induction machine rotates in the same direction as the stator rotating field, the induction machine is in the generating mode of operation. Here, a generating torque acting opposite to the stator rotating magnetic field is produced.
Under known practices, medium voltage drives are remotely mounted separate from rotating electrical apparatus. This requires significant floor space and, sometimes, relatively long lengths of shielded power cables and output filters.
U.S. Pat. No. 6,679,076 discloses that a centrifugal chiller includes both a unit-mounted full-voltage starter and a unit-mounted reduced-voltage starter. The full-voltage starter enclosure is placed in a location that is accessible to a user, adjacent to the motor leads, and convenient for feeding the main power supply lines to the chiller. The full-voltage starter is an electromagnetic on/off switch whose operation determines whether the entire supply line-voltage (e.g., 2,300 volts) or zero voltage is made available to the motor. The reduced-voltage starter can be a primary reactor, an autotransformer, or a solid-state starter. A primary reactor includes a resistor in series with each power line leading to the motor. A set of contacts serves as a shunt across each resistor to effectively add or remove the resistor from its respective line. An autotransformer includes a transformer with multiple leads that a set of contacts selectively taps to apply full-voltage or reduced-voltage across the motor leads. A solid-state starter includes at least one solid-state electrical “switching” device (e.g., SCR, triac, diac, power transistor, etc.) that interrupts or changes the waveform of the power leading to the motor to deliver less power to the motor at startup, and full power afterwards.
U.S. Pat. No. 7,353,662 discloses that a starter box can be replaced with a variable speed drive configured for operation at medium voltage in order to operate a motor at variable speeds, and that the starter box or the variable speed drive is preferably mounted on a chiller system unit with the other components of a chiller system. A solid-state starter device is preferably used to “soft start” the motor on an initial startup of the motor and then to permit operation of the motor at a fixed speed after startup. The solid-state starter device can preferably incorporate semiconductor switches such as silicon controlled rectifiers (SCRs), insulated gate bipolar transistors (IGBTs), diodes or gate turn off (GTO) devices.
FIG. 1A shows a conventional industrial AFD 2 (e.g., 92″ in height) including an input section 4, a transformer and converter section 6, and an inverter section 8. The input section 4 includes an isolation switch (not shown), power fuses (not shown) and a contactor (not shown). The transformer and converter section 6 includes a transformer (not shown) and a converter (not shown). As shown in FIG. 1B, the inverter section 8 includes lower DC link capacitors 10, an intermediate electronic switch (e.g., without limitation, semiconductor switches, such as IGBTs) sub-section 12 above the capacitors 10, and a condenser section of a heat pipe assembly 14 above the intermediate electronic switch sub-section 12.
It is known to provide individual components, such as DC link capacitors, electronic switches, discharge resistors and rectifier diodes, and heat pipe assemblies.
It is also known to mount an input section for an AFD in an enclosure separate from a starter.
Induction motor drives, also called alternating current (AC) drives, are used to control the speed and torque of multiphase induction motors, which for a long time have been the workhorse of the industry.
Today's AC drives can be divided into two categories: low voltage and medium voltage. The low voltage AC drives are widely used and cover the 0 VAC to 600 VAC range. Medium voltage AC drives cover input line voltages above 660 VAC and up to 15,000 VAC. High voltage AC drives cover voltages of 15,000 VAC and higher, but are very uncommon compared to low voltage and medium voltage AC drives.
Until recently, power semiconductor switches were rated at a maximum of 1,700 V, which has allowed the low voltage three-phase AC drives to use a six-switch inverter bridge. Today, state-of-the-art semiconductor switches are rated at 2,500 V, 3,300 V, 4,500 V, 6,500 V and can be used in a two-level six-switch inverter bridge having up to a 2,000 VAC input. Above 2,000 VAC, known inverter bridges employ a greater number of power semiconductor switches connected in series. The most popular inverter topology for three-phase medium voltage induction motors of up to 4,000 V is a three-level twelve-switch inverter bridge.
The number of levels in an inverter bridge defines the number of direct current (DC) voltage steps that are employed by the inverter bridge in order to achieve a certain voltage level in its output. Because power semiconductor switches have limited voltage capability, the total DC bus voltage of an inverter bridge is divided into a number of voltage steps, such that each voltage step can be handled by one power switch.
In a conventional two-level AC drive, three-phase AC power (R, S, T), after passing through an optional input line reactor, is rectified by a rectifier and a capacitor to form a two-level DC bus. Depending on the design approach, input harmonics on the DC bus may be further reduced by a DC reactor. The two-level DC bus voltage is applied across the six-switch inverter bridge which produces a two-level PWM voltage output.
The six switches are divided into three branches with two switches each. A controller controls each switch via the control terminals of the corresponding switch. A three-phase motor has a phase connection derived from the middle point between two switches of a branch, and the three branches produce three phases which collectively drive the motor.
The two-levels of the DC bus constitute a positive bus and a negative bus. The top switch of each branch is connected to the positive bus and the bottom switch is tied to the negative bus. The two switches in a branch are in series and therefore cannot be turned-on at the same time without causing a short-circuit. In order to prevent short-circuit, switch delay times must be taken into consideration by the controller. The top switch needs to turn-off before the bottom one turns-on, and vice-versa. Each of the switches handles the full voltage between the positive and negative busses.
A three-phase inverter bridge has three branches and “L” bus voltage levels (L≧2). Each branch provides one phase of the three-phase output for driving an inductive load. See, for example, U.S. Pat. No. 7,110,272.
An active front end (AFE) converter and an LCL (inductor/capacitor/inductor) filter are known structures.
There is room for improvement in adjustable frequency drives.
There is also room for improvement in packaging of adjustable frequency drive structures.
There is further room for improvement in adjustable frequency drive systems.